FIG. 10 is a schematic cross-sectional view of conventional acoustic wave device 101. Acoustic wave device 101 includes piezoelectric substrate 102, comb-shaped electrode 103 formed on piezoelectric substrate 102, and dielectric film 104 formed above substrate 102 for covering comb-shaped electrode 103. Comb-shaped electrode 103 is configured to exciting a Rayleigh wave having wavelength λ as a main acoustic wave,
Dielectric film 104 having a temperature coefficient of frequency (TCF) having a sign reverse to that of piezoelectric substrate 102 improves the TCF of acoustic wave device 101. A conventional acoustic wave device similar to acoustic wave device 101 is disclosed in, e.g. Patent Literature 1.
Conventional acoustic wave device 101 produces an extraneous acoustic wave, namely, a shear horizontal (SH) wave, having a frequency between a resonance frequency and an anti-resonance frequency of a Rayleigh wave. In the case that acoustic wave device 101 is used in a ladder filter or a double mode SAW (DMS) filter, the SH wave produces a ripple in pass bands of these filters and invites degradation in the characteristics thereof.
FIG. 11 shows admittance characteristics (dB) of acoustic wave device 101. In acoustic wave device 101, piezoelectric substrate 102 is made of a lithium niobate (LiNbO3)-based substrate having a cut surface and a propagation direction of the Rayleigh wave expressed as an Euler angle (φ, θ, ψ), where −10°≦φ≦10°, 33°≦θ≦43°, and −10°≦ψ≦10°. Comb-shaped electrode 103 is made of a molybdenum (Mo) electrode having a film thickness of 0.05λ, and excites a Rayleigh wave having wavelength λ of 4000 nm as a main acoustic wave. Dielectric film 104 is made of silicon dioxide (SiO2) having a film thickness of 0.25λ measured from the interface between substrate 102 and dielectric film 104 to an upper surface of dielectric film 104. As shown in FIG. 11, spurious emission 108 is produced by the SH wave as an extraneous acoustic wave between a resonance point and an anti-resonance point of the Rayleigh wave.